Volumetric Ethanol Productivity Research Articles

Comparative analysis of stirred catalytic basket bio-reactor for the production of bio-ethanol using free and immobilized Saccharomyces cerevisiae cells

The successful industrial production of ethanol and fine chemicals requires the development of new biocatalytic reactors and support materials to achieve economically viable processes. In this work, a Stirred-Catalytic-Basket-BioReactor using various immobilizing foams as support material and compared to free cells were used, focusing mainly on; (i) effect of mass-transfer on cells physiology and (ii) ethanol productivity. The performance of the reactor was further evaluated by ethanol volumetric productivity, yield and time for process completion and it was found that the variation of ethanol production and diffusion of the substrate in fermentation process are co-related with the stirrer speed and initial glucose concentration. It was also observed that the time difference for glucose consumption between free and immobilized cells (alginate and sponges) tends to increase by increasing the glucose concentration in the medium. We found that at higher stirrer speed (500 rpm) when using higher glucose concentration (200 g/l), ethanol volumetric productivity increased significantly in the sponge (85 g/l) as compared to alginate beads (79 g/l) and free cells (60 g/l). From the data obtained, it can be concluded that sponges are the best support material for attaining higher ethanol productivity. A stirred catalytic basket bioreactor with yeast cells immobilized in polyethylene sponge gives higher ethanol production at a higher glucose consumption rate, and this productivity is due to higher mixing efficiency and reduced external as well as internal mass transfer limitations. The potentials of the reactor rank it as a remarkable ethanol/fine-chemical production approach that needs further investigations.

A vertical ball mill as a new reactor design for biomass hydrolysis and fermentation process

A vertical ball mill (VBM) reactor was evaluated for use in biomass conversion processes. The effects of agitation speed (100–200 rpm), number of glass spheres (0–30 units) and temperature (40–46 °C) on enzymatic hydrolysis of rice straw and on glucose fermentation by a thermotolerant Kluyveromyces marxianus strain were separately studied. The results revealed an important role of the spheres during biomass' fiber liquefaction and yeast's fermentative performance. For hydrolysis, the spheres were the only variable with significant positive impact on cellulose conversion, while for fermentation all the variables have influenced the ethanol volumetric productivity (QP). For QP, the spheres showed an interactive effect with temperature, being obtained a maximum of 2.16 g/L.h when both variables were used in the lowest level. By applying the needed adjustments on the levels of the variables for each process (hydrolysis and fermentation), the VBM reactor could be efficiently used for biomass conversion into ethanol.

Industrial robustness linked to the gluconolactonase from Zymomonas mobilis.

The physiological characteristics and the potential gluconolactone production of the gluconolactonase-deficient strain, Zymomonas mobilis ZM4 gnlΔ, were investigated via growth inhibitory assay and biotransformation of glucose and fructose into gluconolactone and sorbitol, respectively. The results of ethanol fermentation studies performed in the presence of high concentration of glucose (>200gl-1) under fermentative or aerobic conditions indicated that a significant reduction of volumetric ethanol productivity from the strain of ZM4 gnlΔ was noticeable due to the reduced rates of specific growth, sugar uptake, and biomass yield as compared with those of the parental strain ZM4. The biotransformation prepared at pH 6.0 using the permeabilized cell indicated that gluconic acid from ZM4 gnlΔ was still produced as a major product (67gl-1) together with sorbitol (65gl-1) rather than gluconolactone after 24h. Only small amount of gluconolactone was transiently overproduced up to 9g l-1, but at the end of biotransformation, all gluconolactone were oxidized into gluconic acid. This indicated that autolysis of gluconolactone at the pH led to such results despite under gluconolactonase inactivation conditions. The physiological characteristics of ZM4 gnlΔ was further investigated under various stress conditions, including suboptimal pH (3.5~6.0), temperature (25~40°C), and presence of growth inhibitory molecules including hydrogen peroxide, ethanol, acetic acid, furfural, and so forth. The results indicated that ZM4 gnlΔ was more susceptible at high glucose concentration, low pH of 3.5, and high temperature of 40°C and in the presence of 4mM H2O2 comparing with ZM4. Therefore, the results were evident that gluconolactonase in Z. mobilis contributed to industrial robustness and anti-stress regulation.

Hydrothermal and Acid Pretreatments Improve Ethanol Production from Lignocellulosic Biomasses

Hydrothermal and acid pretreatments using different acid charges (1.5%, 3.0%, and 4.5% H2SO4) were proposed for eucalyptus, sugarcane bagasse, and sugarcane straw prior to their bioconversion into ethanol using the semi-simultaneous saccharification and fermentation (SSSF) process. The hydrothermal and acid pretreatments were efficient for hemicelluloses removal from eucalyptus (63 to 96%), bagasse (25 to 98%), and straw (23 to 95%) and to remove a substantial amount of lignin from eucalyptus (10 to 34%) and bagasse (10 to 27%). During pretreatments, pseudo-extractives and pseudo-lignin were generated from biomasses. The SSSF was performed in pretreated biomasses using 24 h presaccharification followed by an additional 10 h of simultaneous saccharification and fermentation (SSF). With hydrothermal pretreatment, the eucalyptus presented the highest ethanol production, but only low values for SSSF parameters were obtained, as follows: ethanol yield (0.017 gethanol/gbiomass), volumetric productivity of ethanol (0.16 g L-1 h-1), and ethanol concentration (1.6 g L-1). On the other hand, using acid pretreatment, the straw (pretreated using 4.5% H2SO4) presented the highest ethanol production among the biomasses, assessed based on ethanol yield (0.056 gethanol/gbiomass), volumetric productivity of ethanol (0.51 g L-1 h-1), and ethanol concentration (5.1 g L-1).

Ethanol Production from Water Hyacinth (Eichhornia crassipes) Using Various Types of Enhancers Based on the Consumable Sugars

Ethanol production from cellulosic biomass could effectively increase using enhancers. In this study, five types of different enhancers: fermented malt extract (FME), fermented barley extract (FBE), Saccharomyces cerevisiae (SC), horse dung (HD) and mixture of all enhancers (Mix) were used in order to evaluate the sugar and ethanol production from water hyacinth. Acid pretreatment (H2SO4) was used in order to enhance digestibility for conversion of water hyacinth into ethanol. To observe the efficiency of pretreatment SEM and FTIR was used to observe the structural changes of water hyacinth before and after pretreatment. The sugar consumption rate was in order of MIX, SC, HD, FME and FBE during 60 h fermentation time, respectively. The highest ethanol produced by MIX with 5.1 ± 1.2 g/L with 60% sugar consumption, 0.085 gp/L/h ethanol volumetric productivity, 0.42 gp/gs ethanol yield and 82% theoretical yield in 60 h of fermentation. Ethanol production followed up by 3.4 ± 0.8, 3.2 ± 0.9, 1.5 ± 0.4 and 1.1 ± 0.5 g/L using SC, HD, FME and FBE, respectively. The results shows that ethanol production rate is correlated to sugar consumption during fermentation.

Enhanced xylose fermentation by engineered yeast expressing NADH oxidase through high cell density inoculums.

Accumulation of reduced byproducts such as glycerol and xylitol during xylose fermentation by engineered Saccharomyces cerevisiae hampers the economic production of biofuels and chemicals from cellulosic hydrolysates. In particular, engineered S. cerevisiae expressing NADPH-linked xylose reductase (XR) and NAD+-linked xylitol dehydrogenase (XDH) produces substantial amounts of the reduced byproducts under anaerobic conditions due to the cofactor difference of XR and XDH. While the additional expression of a water-forming NADH oxidase (NoxE) from Lactococcus lactis in engineered S. cerevisiae with the XR/XDH pathway led to reduced glycerol and xylitol production and increased ethanol yields from xylose, volumetric ethanol productivities by the engineered yeast decreased because of growth defects from the overexpression of noxE. In this study, we introduced noxE into an engineered yeast strain (SR8) exhibiting near-optimal xylose fermentation capacity. To overcome the growth defect caused by the overexpression of noxE, we used a high cell density inoculum for xylose fermentation by the SR8 expressing noxE. The resulting strain, SR8N, not only showed a higher ethanol yield and lower byproduct yields, but also exhibited a high ethanol productivity during xylose fermentation. As noxE overexpression elicits a negligible growth defect on glucose conditions, the beneficial effects of noxE overexpression were substantial when a mixture of glucose and xylose was used. Consumption of glucose led to rapid cell growth and therefore enhanced the subsequent xylose fermentation. As a result, the SR8N strain produced more ethanol and fewer byproducts from a mixture of glucose and xylose than the parental SR8 strain without noxE overexpression. Our results suggest that the growth defects from noxE overexpression can be overcome in the case of fermenting lignocellulose-derived sugars such as glucose and xylose.

Development of an acetic acid tolerant Spathaspora passalidarum strain through evolutionary engineering with resistance to inhibitors compounds of autohydrolysate of Eucalyptus globulus

Evolutionary engineering strategy based on mutagenesis by UV irradiation and subsequent selection by continuous cultivation at increasing concentrations of acetic acid in synthetic medium with glucose and xylose mixtures was used to develop an evolved strain of the yeast Spathaspora passalidarum with improved resistance to acetic acid. After 380 generations, the yeast was able to produce 5.8g/L ethanol in the presence of 3.5g/L acetic acid in synthetic medium with mixture of 15gL−1 glucose and 15gL−1 xylose. To demonstrate the improved resistance to acetic acid of the evolved strain compared to the native strain, growth kinetics and bioethanol production of both strains in batch cultures under microaerobic condition were performed. The evolved strain reached an ethanol volumetric productivity of 0.23g/Lh and ethanol yield of 0.48g/g in the presence of 4.5g/L acetic acid. These results were 7-fold and 2-fold higher than those obtained with the native strain, respectively. Inhibitors composition present in Eucalyptus globulus autohydrolysate were (gL−1): acetic acid, 4.7; furfural, 1.0; HMF, 0.36; formic acid, 0.6; syringaldehyde, 0.14 and vanillin, 0.017. When Eucalyptus globulus autohydrolysate was used as culture medium, the evolved strain of S. passalidarum showed complete consumption of glucose and cellobiose, and 56% of xylose. In contrast, the wild type strain was unable to completely consume any of these sugars and showed a lag phase of 46h. In brief, evolutionary engineered strain of S. passalidarum presented an improved resistance to inhibitors usually found in Eucalyptus globulus autohydrolysate and was able to co-ferment glucose, xylose and cellobiose under microaerobic condition without lag phase.

Assessment of alkaline pretreatment for the production of bioethanol from eucalyptus, sugarcane bagasse and sugarcane straw

The impact of alkaline pretreatment at different alkaline charges (5%, 10% and 15% NaOH w/w, on dry basis) on the chemical composition of eucalyptus, sugarcane bagasse and straw were compared with the subsequent bioconversion into ethanol, using semi-simultaneous saccharification and fermentation (SSSF). By increasing alkaline charge in pretreatments, substantial amount of lignin, hemicelluloses and cellulose were fractionated. The chemical composition of biomasses was expressed based on the complete mass balance. The chemical transformation of biomasses during pretreatments was assessed by comparing the chemical composition of pretreated biomasses and their untreated counterparts. Pretreatments promoted delignification in the range of 11%–51% for eucalyptus, 22%–90% for bagasse, and 60%–99% for straw for the alkaline charges in the range of 5%–15% (NaOH w/w). The removal of lignin from bagasse and straw was higher than that from eucalyptus, which was due to the combined effect of higher frequency of both, the free phenolic groups and the ester bonds in grass lignin that made the lignin solubility escalate in alkaline conditions. It was also observed that bagasse had a removal by 37%–45% hemicelluloses and 0.8%–11% cellulose. For straw, higher amount of carbohydrates was removed, in the range of 55%–66% hemicelluloses and 19%–36% cellulose. Fragments of lignin and carbohydrates were converted into new structures called “pseudo-extractives” in eucalyptus during pretreatments. Pseudo-extractives and native extractives were quantified together, thus increasing the total extractives contents by 3.3 (5% NaOH), 3.5 (10% NaOH) and 2.9 (15% NaOH) times, in line with the original raw materials. Maximum ethanol yield and maximum volumetric productivity of ethanol were achieved for eucalyptus pretreated using 10% NaOH, bagasse pretreated using 15% NaOH and straw pretreated using 5% NaOH. At the optimal pretreatment condition, just about 51% glucose was released from pretreated bagasse. Sugarcane bagasse presented the highest values for the respective parameters, namely: 8.8gL−1 ethanol concentration, 0.101gethanol/gbiomass ethanol yield and 0.88gL−1h−1 volumetric productivity of ethanol. Alkaline charge proved to be an important control variable for alkaline pretreatments, with determinant effect on chemical transformations of biomasses and result on ethanol production.

Volumetric oxygen transfer coefficient as a means of improving volumetric ethanol productivity and a criterion for scaling up ethanol production withEscherichia coli

BACKGROUND This study evaluated the influence of different micro-aerated conditions, including the aeration rate, the volumetric oxygen transfer coefficient (kLa) and the oxygen transfer rate (OTR), on improving ethanol productivity and scale-up of the fermentation step of the ethanologenic Escherichia coli strain MS04 in mineral medium supplemented with xylose, glucose, and sodium acetate. RESULTS Growth and ethanol production results using 0.75 L of fermenter showed that micro-aeration (0.1 vvm, 400 rpm) improved the volumetric ethanol productivity and sugar consumption rate compared with the anaerobic condition (0 vvm, 400 rpm) or higher aeration rates (>0.2 vvm) without reducing significantly the ethanol yield. The kLa and the OTR were estimated and a kLa value of 7.2 h−1 was used as a criterion to scale-up the fermentation process from 0.75 L to 9.16 L and 110 L. During scale-up, the volumetric ethanol productivity and ethanol yield of consumed sugars were maintained at similar levels to those obtained in the laboratory in the 0.75 L fermenter. CONCLUSIONS The controlled supply of low levels of oxygen promoted an increase in the concentration of biomass favoring the production and volumetric productivity of ethanol. The use of kLa allowed the fermentation step to be scaled up with ethanologenic E. coli maintaining similar ethanol yields and productivities. © 2016 Society of Chemical Industry

Evaluation of fuel ethanol production from aqueous ammonia-treated rice straw via simultaneous saccharification and fermentation

Rice straw (RS) has been considered a promising feedstock for ethanol production in Asia. However, the recalcitrance of biomass, particularly the presence of lignin, hinders the enzymatic saccharification of polysaccharides in RS and consequently decreases the ethanol yield. Here, we used aqueous ammonia pretreatment to remove lignin from RS (aRS). The reaction conditions were a solid:liquid ratio of 1:12, an ammonia concentration of 27% (w w−1), room temperature, and a 2-week incubation. We evaluated enzymatic digestibility and the ethanol production yield. A 42% reduction in lignin content increased the glucan conversion of aRS to glucose from 20 to 71% using a combination of Cellic Ctec2 cellulases and Cellic Htec2 xylanases at enzyme loads of 15 FPU +100 XU g−1 solid. Scanning electron microscopy highlighted the extensive removal of external fibres and increased porosity of aRS, which aided the accessibility of cellulose for enzymes. Using the same enzyme dosage and a solid load of 100 g L−1, simultaneous saccharification and fermentation using a monoculture of Saccharomyces cerevisiae and co-culture with Candida tropicalis yielded ethanol concentrations of 22 and 25 g L−1, corresponding to fermentation efficiencies of 96 and 86% fermentation, respectively. The volumetric ethanol productivities for these systems were 0.45 and 0.52 g L−1 h−1. However, the ethanol yield based on the theoretical glucose and xylose concentrations was lower for the co-culture (0.44 g g−1) than the monoculture (0.49 g g−1) due to the low xylose consumption. Further research should optimise fermentation variables or select/improve microbial strains capable of fermenting xylose to increase the overall ethanol production yield.

Evaluation of Artichoke Bracts as a Potential Source of Bioactive Compounds, Bio-ethanol Production and Livestock Feeding

Recycling of lignocellulosic agro-industrial residues waste is the key of the environmental sustainability. So, the goal of the present study was to utilize artichoke (Cynara cardunculus L.) bracts as a potential source of bio-active compounds or as a carbon source during bio-production of ethanol and to evaluate its nutritive value for livestock feeding by using in vitro gas production technique. The chemical composition, phenolic compounds, and radicals scavenging property of extracts were determined. Silage production and acid hydrolysis on conversion of lignocellulosicartichoke was studied. Also, the acid hydrolysate of lignocellulosic components as a carbon source was investigatedduring bio-production of ethanol using Saccharomyces cerevisiae and simultaneous saccharification fermentationprocess. The obtained results revealed variation in proximate composition and mineral content among treatments of artichoke bracts used here. The blanching processing of artichoke bracts had higher total phenolic content (935.43 mg GAE /100 g DW) and 89.64% inhibition of DPPH radical with an IC50 value of 6.23 mg/ml. The highest reducing sugar content was obtained by using sulphuric acid (3%) for 20 min at 120°C. Fermentation of the hydrolysates gave the highest ethanol yield of (10.02 g/L), which corresponds to volumetric productivity of ethanol being 1.52 g /L/h with fermentation efficiency of 97.39% and biomass of 4.64 g/L after 48 hr. The results of the present study suggest that adding sugar beet molasses to the fermentation medium enhanced production of bio-ethanol (14.01g/L). The resultsalso showed that the nutritional value of the artichoke bracts is similar to that of good roughage as hay in terms of metabolizable energy (8.42 MJ/kg DM), net energy (3.15MJ/kg DM), short chain fatty acids (99.48 mM), microbial protein synthesis (76.49 g/kg) and organic matter digestibility (63.41 %).

Production of cellulosic ethanol from sugarcane bagasse by steam explosion: Effect of extractives content, acid catalysis and different fermentation technologies

The production of cellulosic ethanol was carried out using samples of native (NCB) and ethanol-extracted (EECB) sugarcane bagasse. Autohydrolysis (AH) exhibited the best glucose recovery from both samples, compared to the use of both H3PO4 and H2SO4 catalysis at the same pretreatment time and temperature. All water-insoluble steam-exploded materials (SEB-WI) resulted in high glucose yields by enzymatic hydrolysis. SHF (separate hydrolysis and fermentation) gave ethanol yields higher than those obtained by SSF (simultaneous hydrolysis and fermentation) and pSSF (pre-hydrolysis followed by SSF). For instance, AH gave 25, 18 and 16gL−1 of ethanol by SHF, SSF and pSSF, respectively. However, when the total processing time was taken into account, pSSF provided the best overall ethanol volumetric productivity of 0.58gL−1h−1. Also, the removal of ethanol-extractable materials from cane bagasse had no influence on the cellulosic ethanol production of SEB-WI, regardless of the fermentation strategy used for conversion.

The Presence of Pretreated Lignocellulosic Solids from Birch during Saccharomyces cerevisiae Fermentations Leads to Increased Tolerance to Inhibitors--A Proteomic Study of the Effects.

The fermentation performance of Saccharomyces cerevisiae in the cellulose to ethanol conversion process is largely influenced by the components of pretreated biomass. The insoluble solids in pretreated biomass predominantly constitute cellulose, lignin, and -to a lesser extent- hemicellulose. It is important to understand the effects of water-insoluble solids (WIS) on yeast cell physiology and metabolism for the overall process optimization. In the presence of synthetic lignocellulosic inhibitors, we observed a reduced lag phase and enhanced volumetric ethanol productivity by S. cerevisiae CEN.PK 113-7D when the minimal medium was supplemented with WIS of pretreated birch or spruce and glucose as the carbon source. To investigate the underlying molecular reasons for the effects of WIS, we studied the response of WIS at the proteome level in yeast cells in the presence of acetic acid as an inhibitor. Comparisons were made with cells grown in the presence of acetic acid but without WIS in the medium. Altogether, 729 proteins were detected and quantified, of which 246 proteins were significantly up-regulated and 274 proteins were significantly down-regulated with a fold change≥1.2 in the presence of WIS compared to absence of WIS. The cells in the presence of WIS up-regulated several proteins related to cell wall, glycolysis, electron transport chain, oxidative stress response, oxygen and radical detoxification and unfolded protein response; and down-regulated most proteins related to biosynthetic pathways including amino acid, purine, isoprenoid biosynthesis, aminoacyl-tRNA synthetases and pentose phosphate pathway. Overall, the identified differentially regulated proteins may indicate that the likelihood of increased ATP generation in the presence of WIS was used to defend against acetic acid stress at the expense of reduced biomass formation. Although, comparative proteomics of cells with and without WIS in the acetic acid containing medium revealed numerous changes, a direct effect of WIS on cellular physiology remains to be investigated.

Continuous Ethanol Production from D-xylose: Using Free Cells of Clavispora opuntiae

The continuous fermentation of D-xylose to ethanol by free cells of Clavispora opuntiae was studied in a continuous stirred tank reactor at 25 ± 0.5°C and pH 4.2 ± 0.2. The maximum 89.8 ± 0.04% ethanol yield was obtained as compared to theoretical yield and about 96.00 ± 0.05% sugar was utilized at a dilution rate of 0.014 h–1. The maximum values of volumetric ethanol (Qp) and biomass (Qx) productivity were found to be 0.67 ± 0.04 gp l–1 h–1 and 0.18 ± 0.007 gx l–1 h–1, respectively, at a dilution rate of 0.09 h–1. The maintenance energy coefficient for C. opuntiae was found to be 0.21 ± 0.005 gs gx–1 h–1, whereas the values of specific ethanol production rate (qp) and specific sugar uptake rate (qs) were found to increase almost linearly with the increase of dilution rate. The maximum specific ethanol productivity (qp max) and maximum specific sugar uptake rate (qs max) were 0.50 ± 0.004 gp gx–1 h–1 and 4.00 ± 0.006 gs gx–1 h–1, respectively.

Continuous ethanol production from sugarcane bagasse hydrolysate at high temperature with cell recycle and in-situ recovery of ethanol

A continuous process with cell recycle was demonstrated on fermentation of sugarcane bagasse hydrolysate to ethanol having a facility of in-situ stripping of ethanol by air or nitrogen gas. The thermotolerant yeast Kluyveromyces sp. IIPE453, grown in xylose-rich hydrolysate, was used in this study for fermenting the glucose-rich hydrolysate at 50°C. The effect of dilution rate, cell mass concentration, and air or nitrogen gas stripping on ethanol production was evaluated. The maximum volumetric productivity of ethanol was 1.22gl−1h−1 on stripping the bioreactor with nitrogen at a dilution rate of 0.075h−1. An ethanol yield of 96% of its theoretical value and 95% sugar conversion efficiency could be obtained while the ethanol concentration in the condensate of the stripper was found to be 56.3gl−1, which was almost 3.4 times concentrated as compared to ethanol concentration in the broth without stripping. The specific ethanol productivity could be improved due to increase in sugar conversion efficiency and higher ethanol yield in a continuous cell recycle fermentation system applying gas stripping at high temperature.

Continuous Ethanol Fermentation of Pretreated Lignocellulosic Biomasses, Waste Biomasses, Molasses and Syrup Using the Anaerobic, Thermophilic Bacterium Thermoanaerobacter italicus Pentocrobe 411

Lignocellosic ethanol production is now at a stage where commercial or semi-commercial plants are coming online and, provided cost effective production can be achieved, lignocellulosic ethanol will become an important part of the world bio economy. However, challenges are still to be overcome throughout the process and particularly for the fermentation of the complex sugar mixtures resulting from the hydrolysis of hemicellulose. Here we describe the continuous fermentation of glucose, xylose and arabinose from non-detoxified pretreated wheat straw, birch, corn cob, sugar cane bagasse, cardboard, mixed bio waste, oil palm empty fruit bunch and frond, sugar cane syrup and sugar cane molasses using the anaerobic, thermophilic bacterium Thermoanaerobacter Pentocrobe 411. All fermentations resulted in close to maximum theoretical ethanol yields of 0.47–0.49 g/g (based on glucose, xylose, and arabinose), volumetric ethanol productivities of 1.2–2.7 g/L/h and a total sugar conversion of 90–99% including glucose, xylose and arabinose. The results solidify the potential of Thermoanaerobacter strains as candidates for lignocellulose bioconversion.

Effect of nutrient supplementation on ethanol production in different strategies of saccharification and fermentation from acid pretreated rice straw

The effect of nutrient supplementation on ethanol production by recently selected thermotolerant yeast (Kluyveromyces marxianus NRRL Y-6860) was investigated in different strategies of saccharification and fermentation employing rice straw pretreated by dilute acid. Among the evaluated strategies, similar ethanol yields (YP/S ∼ 0.23 g g−1) were obtained with or without nutrient addition. However, considering the whole process time, the strategy based on simultaneous saccharification and fermentation (SSF), without pre-hydrolysis, was assigned as the most suitable configuration due to the highest ethanol volumetric productivity (1.4 g L−1 h−1), about 2-fold higher in relation to the others. The impact of enzymatic preparation employed in this study was also evaluated on glucose fermentation in semi-synthetic medium. The enzymatic preparation affected both glucose consumption and ethanol production by K. marxianus NRRL Y-6860, but just in the absence of nutrients. Therefore, the enzyme type and loading should be carefully defined, not only by the capital costs involved, but also by the possibility of increasing the fermentation inhibitors.

Effects of aeration on growth, ethanol and polyol accumulation by Spathaspora passalidarum NRRL Y-27907 and Scheffersomyces stipitis NRRL Y-7124.

Spathaspora passalidarum NN245 (NRRL-Y27907) is an ascomycetous yeast that displays a higher specific fermentation rate with xylose than with glucose. Previous studies have shown that its capacity for xylose fermentation increases while cell yield decreases with decreasing aeration. Aeration optimization plays a crucial role in maximizing bioethanol production from lignocellulosic hydrolysates. Here, we compared the kinetics of S. passalidarum NN245 and Scheffersomyces (Pichia) stipitis NRRL Y-7124 fermenting 15% glucose, 15% xylose, or 12% xylose plus 3% glucose under four different aeration conditions. The maximum specific fermentation rate for S. passalidarum was 0.153 g ethanol/g CDW · h with a yield of 0.448 g/g from 150 g/L xylose at an oxygen transfer rate of 2.47 mmol O2 /L h. Increasing the OTR to 4.27 mmol O2 /L h. decreased the ethanol yield from 0.46 to 0.42 g/g xylose while increasing volumetric ethanol productivity from 0.52 to 0.8 g/L h. Both yeasts had lower cells yields and higher ethanol yields when growing on xylose than when growing on glucose. Acetic acid accretions of both strains correlated positively with increasing aeration. S. passalidarum secreted lower amounts of polyols compared to S. stipitis under most circumstances. In addition, the composition of polyols differed: S. passalidarum accumulated mostly xylitol and R,R-2,3-butanediol (BD) whereas S. stipitis accumulated mostly xylitol and ribitol when cultivated in xylose or a mixture of 12% xylose and 3% glucose. R,R-2,3-BD accumulation by S. passalidarum during xylose fermentation can be as much as four times of that by S. stipitis, and R,R-2,3-BD is also the most abundant byproduct after xylitol. The ratios of polyols accumulated by the two species under different aeration conditions and the implications of these observations for strain and process engineering are discussed.

Fermentation of lignocellulosic hydrolyzate using a submerged membrane bioreactor at high dilution rates.

A submerged membrane bioreactor (sMBR) was developed to ferment toxic lignocellulosic hydrolyzate to ethanol. The sMBR achieved high cell density of Saccharomyces cerevisiae during continuous cultivation of the hydrolyzate by completely retaining all yeast cells inside the sMBR. The performance of the sMBR was evaluated based on the ethanol yield and productivity at the dilution rates 0.2, 0.4, 0.6, and 0.8h(-1) with the increase of dilution rate. Results show that the yeast in the sMBR was able to ferment the wood hydrolyzate even at high dilution rates, attaining a maximum volumetric ethanol productivity of 7.94 ± 0.10 g L(-1)h(-1) at a dilution rate of 0.8h(-1). Ethanol yields were stable at 0.44 ± 0.02 g g(-1) during all the tested dilution rates, and the ethanol productivity increased from 2.16 ± 0.15 to 7.94 ± 0.10 g L(-1)h(-1). The developed sMBR systems running at high yeast density demonstrate a potential for a rapid and productive ethanol production from wood hydrolyzate.

Effectiveness of dilute oxalic acid pretreatment of Miscanthus × giganteus biomass for ethanol production

In the present study the effect of temperature, reaction time and dilute oxalic acid (OA) concentration during steam-pretreatment of Miscanthus × gigantueus has been evaluated using the combined severity factor (CS). At the highest CS glucan and lignin content in the water insoluble fraction (WIF) increased, while xylan content decreased. While glucose recovery in the water soluble fraction (WSF) was found at low concentration when mild CS were used (≤5.0 g L−1 at CS ≤ 2.17), xylose and arabinose concentrations were higher at low-mild CS (1.58–2.17) with a concentration peak at CS 2.03 (39.9 and 3.2 g L−1 for xylose and arabinose, respectively). The decrease in pentoses coincided with inhibitory formation in the WSF, namely acetic acid, furfural, HMF and phenolic compounds. Glucan conversion rose from 46.1% at CS 1.54 to 91.2% at CS 2.76. Likewise, maximum ethanol concentration was achieved at CS 2.76, corresponding to 20.2 g L−1 and a volumetric ethanol productivity of 0.28 g L−1 h−1. Negative correlations have been found between xylan vs. glucan conversion and xylan vs. ethanol production, suggesting that decreasing the xylan content in WIF increases both saccharification rate and ethanol concentration (R2 0.91 and R2 0.93, respectively). On the other hand, a positive correlation was found between ethanol production and glucan conversion (R2 0.93). Fermentation of WSF by Scheffersomyces (Pichia) stipitis CBS 6054 at CS 1.54 produced 12.1 g L−1 of ethanol after 96 h incubation with a volumetric ethanol productivity of 0.13 g L−1 h−1.